Method for separating micro-organisms from earth samples



United States Patent 3,451,545 METHOD FOR SEPARATING MICRO-ORGANISMS FROM EARTH SAMPLES Heinz P. Studer, Bellaire, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed July 13, 1967, Ser. No. 653,011

Int. Cl. B03c 1/00; A01n 1/00; C23c 17/00 U.S. Cl. 209-8 9 Claims ABSTRACT OF THE DISCLOSURE A method for coating micro-organisms such as foraminifera in earth samples with a magnetizable compound by treating the foraminifera with an aqueous solution containing a soluble salt of a complex of a magnetizable metal and a polyfunctional acidic material and thereby depositing a magnetic compound on the foraminifera. The foraminifera can then be separated from the remaining earth samples by magnetic separating means.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to an improved method for isolating micro-organisms such as foraminifera from earth samples; more particularly, it relates to an improved method for coating the foraminifera with magnetic material so that the foraminifera can be separated from the remaining earth samples by magnetic separating means.

Description of the prior art Micropaleontological analysis is an especial tool of the geologist by which he establishes stratigraphic correlations and maps subsurface structures. These analyses furnish information about the nature of the formations traversed, the quality of the fluids contained therein, their pressure, temperature and other bottom hole data. Such information is of great value in many phases of petroleum engineering-exploration, exploitation and reservoir operations.

By obtaining foraminifera samples from close and regular intervals from the earth, as, for example, from a well, so as to give as nearly a continuous stratigraphical sequence as possible, the paleontologist can determine the stratigraphical horizon of the scattered material. Micropaleontology is very largely concerned with the study of classification of foraminifera (or forams), microscopic one-celled organisms which have left their minute tests buried in the rocks as a record of their former existence. Their little shells are most likely to be found in muds, clays and shales, that is, in the finer sediments which were deposited in water that was not too violently agitated nor too swiftly flowing. Like all fossils, some species are found ranging through several or many geologic formations, whereas others are more narrowly limited. Formations may sometimes be recognized and distinguished by a characteristic species, or more often by a particular assemblage of species, different from the species in other associated formations.

It is very important to obtain foram samples that are as nearly perfect as possible. One of the most time-consuming operations in routine micropaleontology is the separation of microfossils from the washed residue of a sample of an earth formation. When hollow and filled with air, forams may be concentrated with heavy liquids, such as carbon tetrachloride or bromoform. Unfortunately, in most cases it has been necessary to pick out the forams by hand.

In one method, forams are separated from earth sam- 3,451,545 Patented June 24, 1969 ples by treating the earth samples with a 2 to 5 percent solution of ferric chloride. A coating of an iron oxide hydrate is thus deposited on the foram samples. This coating, although extremely thin, is sufficient to allow magnetic separation of the coated foram samples by magnetic separation means. However, the ferric chloride solution is a relatively strong acidic solution that severely attacks the carbonate fossils and can damage the fragile foraminifera.

SUMMARY OF THE INVENTION The invention is an improvement in the method of soaking foraminifera obtained from earth formations, such as a well borehole, in ferric chloride solution to deposit magnetic iron compounds on the forams. The improved method herein disclosed rapidly deposits iron hydroxides on the carbonate shells of the forams while dissolving only very small amounts of carbonate. The improvement comprises contacting the forams with an aqueous solution containing a soluble salt of a complex of at least one magnetizable metal and a polyfunctional acidic material at a pH of incipient precipitation of hydrated oxides of the metal. In a preferred embodiment, a solution having a molar concentration of .1 mole of ferric salt and about .3 mole of malonic acid and neutralized to a pH of 5.5 to 6.0 was found to give the best results in terms of a relatively high iron deposition rate and a relatively low rate of carbonate dissolution.

It is an object of this invention to enable large numbers of earth samples in diflicult paleontological problems, such as those arising in correlations in the Plio- Pleistocene formation of offshore Louisiana, to be easily and quickly handled.

It is a further object to materially simplify the separation of foraminifera from samples of bit cuttings from well boreholes.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention utilizes selected chelating materials that keep magnetizable metal ions, such as ferric ions, in solution as complexes in mildly acidic solutions that do not damage foraminifera. Earth samples containing micro-organisms as foraminifera are first obtained from an underground formation, as, for example, from a well borehole by substantially any sampling means, of Which many are known.

The method of separating the forams from earth samples is accomplished by treating the earth samples with a solvent solution containing weak complexes resulting from the reaction of salts of elements susceptible to magnetization, e.g., Fe, Ni, Co, rare earths, etc., and a polyfunctional acidic compound capable of weakly complexing with the salt. The weak complexes become instable in the local increases in pH in the portions of the solutions in contact with the surfaces of the forams. This releases the magnetizable elements and causes magnetizable materials to be deposited on the forams. The samples thus treated are removed from the solution, Washed, and dried and the coated forams are separated from the remainder of the sample by suitable means such as magnetization.

To prevent separation or losses due to the floating of the forams from the samples, the earth samples are preferably first treated with a wetting agent, such as one or more alcohols, glycols, etc.

Useful magnetizable metal salts are iron, nickel, cobalt, rare earth, chloride, nitrate, sulfate, acetate, etc., as well as mixtures of one or more of such salts. The complexing polar compounds include polyfunctional acidic compounds of dibasic acids such as oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic, sebasic acids;

hydroxy acids, e.g., hydroxy acetic, hydroxy propionic, hydroxy butyric, malic, tartaric, citric acids; amino acids and mixtures thereof.

Particularly useful complexes result from the reaction in an aqueous medium of a ferric salt and malonic acid to which suflicient sodium hydroxide or other alkali metal or ammonium hydroxide is added to form an alkali metal salt of a ferric malonate complex. In a preferred embodiment, three parts of malonic acid are mixed with one part of an aqueous solution of a ferric salt. This ratio is preferred as giving optimum results; however, ratios in the range of one part ferric salt and two parts malonic acid to one part ferric salt and four parts malonic acid have been used with varying degrees of success. In each case, the pH of the aqueous solution containing the ferric malonate complex is adjusted, as, for example, by adding a sufl'lcient amount of a caustic alkali such as sodium hydroxide to the point of incipient precipitation of ferric hydroxide. A solution having a pH of 5.5 to 6. is preferably used to give the best results in terms of high iron deposition rate and low rate of carbonate dissolution.

The concentration of the aqueous solution containing the ferric malonate complex is not critical since rapid deposition of iron salts occurs over a range of iron concentrations from .05 to 0.5 M. Solutions of high concentration, as, for example, .5 M FeCl with 1.5 M malonic acid, however, tend to float the foraminifera. A solution of about 0.1 molar concentration of iron is thus preferred with the pH of the .l molar concentration solution preferably maintained at least as high as 5.5 to minimize dissolution of the carbonate. The pH of the solution is also maintained below 6.0 to obtain rapid deposition of iron hydroxides on the foram samples. Thus, the pH at which initial precipitation of Fe(OH) occurs depends largely on the molar ratio of Fe/malonic acid, and only to a small extent on the actual concentration of the solution. The earth samples containing the foraminifera are contacted with the aqueous solution that contains a sodium salt of the ferric malonate complex by any known means.

Foram-containing earth samples are preferably first wetted with a preferentially water-wetting agent, e.g., Cellosolve or alcohol, and then treated with a solution containing a complex of a magnetizable element, as noted above. The Wetting agent prevents some forams" from floating on top of the solution and also aids in uniformly coating the forams with the magnetizable compound that is released from the complex. The earth samples are heated at approximately 80 C. for five to ten minutes, in order to produce a proper coating. After the forams have been coated, the earth samples are washed and dried, as, for example, by heating them at 110 to 120 C., and the dried earth samples are ready for magnetic separation.

In the case of the above illustrative example, the coated forams can be removed from the dried earth sample by use of a strong magnetic field such as that used in a Frantz Iso-dynamic Magnetic Separator. However, it is preferable to subject the dried material to a reduction process by heating it at about 400 C. in the presence of hydrogen so as to convert the coating material to more readily magnetizable lower iron oxides and in this form magnetic separation techniques are more effective. This reduction treatment does not damage the coated forams for examination by micropaleontologists for study under light microscopes, scanning electron microscopes, photographic means and the like.

Magnetic separators, which are well known in the art, are used to separate the coated foraminifera from the remainder of the earth samples. There is very little damage to the forams since the iron-containing complexes were dissolved in a mildly acidic solution that dissolves only insignificant amounts of carbonate from the forams.

4 A solution having a molar ratio of iron to malonic acid of 1:3 is preferred; however, varying degrees of success have been obtained with molar ratios from 1:2 to 1:4 with iron molar concentrations from .1 to .4 grams and pH factors from 2.4 to 6.6. The mixtures of the forams and such a solution of a complex of at least one magnetizable metal and a polyfunctional acidic material are preferably heated at about C. for from about 2 to 10 minutes, although lower temperatures and longer times or higher temperatures and shorter times can be used.

I claim:

1. An improved method for coating micro-organisms disposed in earth samples with magnetic material so that the micro-organisms can be isolated by magnetic separating means, the method comprising treating the earth samples containing micro-organisms in a solution containing a soluble salt of a complex of a magnetizable metal and a polyfunctional acidic material, the magnetizable metal being selected from the group consisting of iron, cobalt, nickel, and rare earths, the polyfunctional acidic material being a dibasic acid and the soluble salt being selected from the group consisting of alkali metal and ammonium salts.

2. An improved method for coating micro-organisms disposed in earth samples with magnetic material so that the micro-organisms can be isolated by magnetic separating means, the method comprising treating the earth samples containing micro-organisms in a solution containing a soluble salt of a complex of a magnetizable metal and a polyfunctional acidic material, the soluble salt being selected from the group consisting of alkali metal and ammonium salts and the complex being derived from the reaction of a ferric salt, malonic acid and an alkali hydroxide and the pH of the solution being between 5.5 and 6.0.

3. The method of claim 2 including the step of establishing a molecular ratio of substantially one part of ferric salt to three parts of malonic acid.

4. The method of claim 3 including the step of heating the samples with the complex solution at a temperature of approximately 80 C. for approximately 5 to 10 minutes to produce proper coating.

5. The method of claim 4 including the step of pretreating the earth samples with a water-wettable agent.

6. The method of claim 2 including the step of establishing a solution of approximately .1 moles of ferric salt to .3 mole of malonic acid.

7. The method of claim 1 including the step of applying magnetic separating means to the earth samples for segregating the coated micro-organisms from the remainder of the earth samples.

8. The method of claim 2 including the steps of drying the coated earth samples and reducing the coating on the micro-organisms.

9. The method of claim 1 including the step of heating the coated micro-organisms at approximately 400 C. in the presence of hydrogen so as to convert the coating to magnetizable lower iron oxides.

References Cited UNITED STATES PATENTS 519,902 5/1-894 Barton 209-8 1,405,690 2/1922 Herr 209-8 2,460,188 1/ 1949 OKane 117-3 2,971,9 16 2/ 196 1 Schleicher.

HARRY B. THORNTON, Primary Examiner.

ROBERT HALPER, Assistant Examiner.

US. 01. X.R. 

